The present invention relates to a process for the separation of liquid hydrocarbons from a particulate Fischer-Tropsch catalyst.
In the Fischer-Tropsch reaction a gaseous mixture of carbon monoxide and hydrogen (synthesis gas) is reacted in the presence of a catalyst to give a hydrocarbon mixture having a relatively broad molecular weight distribution. This product is predominantly straight chain, saturated liquid hydrocarbons which typically have a chain length of more than 5 carbon atoms. Fischer-Tropsch processes may be operated using slurry systems which employ a suspension of catalyst particles in a liquid medium. In a slurry process it is necessary to separate the liquid hydrocarbon products of the Fischer-Tropsch synthesis reaction from the Fischer-Tropsch catalyst.
PCT patent application number WO 98/50492 relates to a process for filtering hydrocarbon liquid from a three phase hydrocarbon slurry comprising gas bubbles and particulate catalyst solids in a hydrocarbon liquid, wherein the slurry is fed through a hydrocarbon liquid filtration zone by means of a gas disengaging downcomer immersed in the slurry, so that it contacts the filter under flow conditions. The gas disengaging downcomer produces a gas reduced, densified slurry and passes it to a filtration zone either inside or outside the slurry reactor. The gas reduced and densified slurry flows past and contacts the filter under relatively high net flow conditions in a net single direction as filtration occurs. This is said to reduce the buildup of catalyst particles as filter cake on the filtration surface of the filter, due to the shearing, scouring and removing action of the flowing slurry. The process of WO 98/50492 relies on a difference in density between the gas reduced slurry and the slurry body in the reactor to circulate slurry through the filtration zone. It has now been discovered that it is possible to feed slurry through a filtration unit by means of a mechanical pump, for example, an impeller, propeller or the like. This is surprising since WO 98/50492 teaches against using mechanical means, because they will quickly erode and cause attrition of the catalyst particles. Furthermore, the process of the present invention allows the suspension to be filtered without first having to reduce the gas content of the slurry.